In recent years, for a radio communication, various distortion compensation methods have been proposed to compensate for a distortion generated in a power amplifier. One of the above-described methods is performed to compensate for a distortion, which is generated in a power amplifier, by approximating, by a polynomial, reverse characteristics of input/output characteristics of the power amplifier and using the polynomial. Specifically, according to the above-described distortion compensation method, the distortion generated in the power amplifier is compensated by using a polynomial in which an instantaneous value of an input signal to be input into the power amplifier is raised to the power of a maximum degree N (N is an integer larger than 0).
The distortion generated in the power amplifier may be affected by a so-called memory effect. Therefore, in this case, a distortion component generated in an output signal from the power amplifier has a frequency characteristic in which the signal is varied according to a past input signal. According to the above-described distortion compensation method using the polynomial in which simply the instantaneous value of the input signal is considered, the distortion component with the frequency characteristic is difficult to be compensated.
Recently, there has been a proposal of a distortion compensation method using a polynomial in which an input signal in various times is considered so that the distortion component with the frequency characteristic is compensated. Specifically, the above-described distortion compensation method uses a polynomial in which the input signal to be input into the power amplifier is raised to the power of the maximum degree N (N is an integer larger than 0) and the raised signal is delayed by the maximum delay number K. According to the polynomial, an input signal may be expressed in a time earlier by a K sample at the largest than the current sample time, so that a desired frequency characteristic may be attached to the input signal. Therefore, according to the distortion compensation method, the distortion component with the frequency characteristic may be compensated.